The products of the I region of the major histocompatibility complex, I region associated (Ia) molecules, are intimately involved with, if not identical to, the genes controlling specific immune responses (Ir). I propose to study the way in which Ia molecules on antigen presenting cells (APC) interact with antigen to determine the immune response pattern of antigen-specific T cells by the preparation of specific Ia mutants in functional antigen presenting cells (APC). We have recently prepared somatic cell hybrids between normal heterozygous B cells and a drug-marked variant of a BALB/c B lymphoma cell line. These hybrid cell lines have potent APC ability and express the Ir gene patterns of both the normal B cell and the tumor cell partners. We now hope to obtain Ia mutants of these cell lines by first mutagenizing the cells with ethylmethane sulfonate, then by immunoselecting with a monoclonal anti-Ia reagent and complement and finally screening the remaining cells on the fluorescence activated cell sorter (FACS) with a second monoclonal anti-Ia antibody directed at a distinct but closely proximate Ia determinant. Cells which have lost the expression of the first Ia determinant but which retain the second one may be assumed to have undergone point mutations. It is hoped that by using a panel of different anti-Ia monoclonal antibodies for both the initial selection step and for the screening on the FACS a range of point mutations may be obtained. In addition, we may also obtain mutants which have lost large segments of or the entire I region. These mutants will then be characterized functionally by correlating the loss of certain Ia determinants with the loss of the ability to present specific foreign antigens to cloned T cell lines, to antigen-specific T cell hybridomas and to freshly isolated primed T cells. These experiments should increase our understanding of the role of Ia molecules in antigen presentation and more specifically may allow us to assign the presentation of specific antigens to well-defined Ia antigenic determinants.